1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an apparatus for hardening sealant. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for eliminating substrate defects in fabricating an LCD device.
2. Discussion of the Related Art
A thin flat panel display tends to have a thickness of no more than a few centimeters. Particularly, a liquid crystal display (LCD) has a wide scope of applications, such as notebook computers, computer monitors, gauge monitors for spacecrafts and aircrafts, and the like.
In general, the LCD is provided with a lower substrate having thin film transistors and pixel electrodes formed thereon, an upper substrate opposite to the lower substrate having a black matrix (BM), a color filter layer, and a common electrode, which are formed thereon, and a liquid crystal layer between the two substrates, for driving the liquid crystal by the electric field generated by the power supply applied to the pixel electrodes and the common electrode between the substrates, to regulate the transmittivity of the liquid crystal, thereby displaying a picture on the display screen.
In the foregoing LCD, a vacuum injection method and a liquid crystal dropping method have been used for forming a liquid crystal layer between the lower substrate and the upper substrate.
A method for fabricating a liquid crystal display using the vacuum injection method will now be explained.
A lower substrate and an upper substrate are prepared for the process. The lower substrate includes a thin film transistor and a pixel electrode. And, the upper substrate includes a black matrix, a color filter layer, and a common electrode.
In addition, a plurality of spacers are formed on either one of the substrates so as to maintain a uniform cell gap between the two substrates. The spacers are either formed of a plurality of ball spacers, which are spread on the substrate, or a plurality of column spacers, which are attached and fixed to the substrate.
A sealant preventing the liquid crystal from leaking and bonding the two substrates is deposited onto one of the substrates. Herein, a thermo-hardening sealant based on an epoxy resin is used as a sealant.
After attaching the two substrates, the thermo-hardening sealant is heated and hardened, thereby bonding the two substrates. Subsequently, the bonded substrates are placed inside a vacuum chamber. A space between the two substrates is maintained under a vacuum condition and immersed into a liquid crystal container, thereby forming a liquid crystal layer inside the bonded substrates.
However, with the advent of large-sized display screens, the method for fabricating a liquid crystal display using the vacuum injection method has become disadvantageous. More specifically, the time period for injecting the liquid crystal has become longer, thereby decreasing productivity.
A liquid crystal dropping method is used to resolve such problems caused by the vacuum injection method. The steps of preparing a lower substrate and an upper substrate, forming a plurality of spacers, and forming a sealant are the same as those in the vacuum injection method.
Subsequently, unlike the vacuum injection method whereby a liquid crystal is injected into the bonded substrates maintained at a vacuum state, a plurality of liquid crystal droplets are dispensed on the lower substrate where the sealant is formed. Then, the two substrates are bonded together.
In the liquid crystal dropping method, the droplets of liquid crystal are first dispensed on the lower substrate, the bonding of the two substrates is carried out thereafter. In this case, when a thermo-hardening sealant is used, the sealant may leak during the heating process and contaminate the liquid crystal. Therefore, in the liquid crystal dropping method, an ultra-violet (UV) hardening sealant based on an acrylic resin is used as the sealant.
Therefore, after attaching the two substrates, UV-rays are irradiated so as to bond the substrates. Also, when a UV hardening sealant partially including an epoxy resin is used, a heating process has to be further performed after irradiating the UV-rays, so as to completely harden the sealant.
The apparatus for hardening a sealant of the related art, more particularly, hardening a thermo-hardening sealant based on an epoxy resin in the vacuum injection method and the liquid crystal dropping method is shown in FIG. 1A.
FIG. 1A is a schematic perspective view of the related art apparatus for hardening a sealant. As shown in FIG. 1A, the related art apparatus for hardening a sealant includes a chamber 1, and a plurality of supporting bars 3 fixed on both inner sides of the chamber 1.
Bonded substrates each having a sealant are loaded upon the supporting bars 3. And, at least two supporting bars 3 form a step. A plurality of these steps are formed to load the substrates thereon.
The inside of the chamber 1 is heated by introducing heated air. The heated air is introduced through an introduction hole (not shown). In addition, a door (not shown) is formed on the side surface of the chamber 1, to which the supporting bars 3 are fixed. The door is opened when the bonded substrates are loaded upon and unloaded from the supporting bars, and closed when the heated air is introduced into the chamber 1.
A method for hardening a sealant using the apparatus for hardening a sealant having the above-described structure will be described. The door (not shown) on the chamber 1 is opened, so as to move bonded substrates by using a robot arm and to load the bonded substrates upon the supporting bars 3 inside the chamber 1. Then, the door is closed, and heated air is introduced into the chamber 1 so as to harden the sealant. Subsequently, the door is opened and the bonded substrates are unloaded.
FIG. 1B illustrates a perspective view showing the bonded substrate 5 being loaded upon the supporting bars 3 inside the chamber 1. FIG. 1C is a cross-sectional view taken along line IC-IC of FIG. 1B. As shown in FIGS. 1B and 1C, the supporting bars 3 of the related art apparatus for hardening a sealant has a round cross-section, thereby allowing the bonded substrate 5 and the supporting bars 3 to come into a linear contact.
Therefore, the region where the bonded substrate 5 coming into a linear contact with the supporting bars 3 is pressed by the heavy weight of the substrate, thereby pressing column spacers formed in the region. This pressing effect results in an image deficiency, whereby black spots are formed when the image is displayed on the screen.
In addition, when the bonded substrates 5 are heated at a high temperature during the hardening process of the sealant, the physical properties of the bonded substrates 5 may be altered. In this case, due to a small contact area between the supporting bars 3 and the bonded substrates 5, a region of the bonded substrates 5 between the supporting bars 3 may droop or sag, as shown in FIG. 1C.